First Optical Hyperfine Structure Measurement in an Atomic Anion

We have investigated the hyperfine structure of the transition between the 5d⁷6s^{2 4}F_9/2^e ground state and the 5d⁶6s²6p ⁶D_J^o excited state in the negative osmium ion by high-resolution collinear laser spectroscopy. This transition is unique because it is the only known electric-dipole transition in atomic anions and might be amenable to laser cooling. From the observed hyperfine structure in ¹⁸⁷Os^- and ¹⁸⁹Os^- the yet unknown total angular momentum of the bound excited state was found to be J=9/2. The hyperfine structure constants of the ⁴F_9/2^e ground state and the ⁶D_9/2^o excited state were determined experimentally and compared to multiconfiguration Dirac-Fock calculations. Using the knowledge of the ground and excited state angular momenta, the full energy level diagram of ¹⁹²Os^- in an external magnetic field was calculated, revealing possible laser cooling transitions.